Free Fructose Is Conformationally Locked

Fructose has been examined under isolation conditions using a combination of UV ultrafast laser vaporization and Fourier-transform microwave (FT-MW) spectroscopy. The rotational spectra for the parent, all (six) monosubstituted ¹³C species, and two single D species reveal unambiguously that the free hexoketose is conformationally locked in a single dominant β-pyranose structure. This six-membered-chair skeleton adopts a ²C₅ configuration (equivalent to ¹C₄ in aldoses). The free-molecule structure sharply contrasts with the furanose form observed in biochemically relevant polysaccharides, like sucrose. The structure of free fructose has been determined experimentally using substitution and effective structures. The enhanced stability of the observed conformation is primarily attributed to a cooperative network of five intramolecular O–H···O hydrogen bonds and stabilization of both endo and exo anomeric effects. Breaking a single intramolecular hydrogen bond destabilizes the free molecule by more than 10 kJ mol^–1. The structural results are compared to ribose, recently examined with rotational resolution, where six different conformations coexist with similar conformational energies. In addition, several DFT and ab initio methods and basis sets are benchmarked with the experimental data